In addition to good advice by Lurker,don't overlook possibility of defective thermostate causing overheating and resultant T&P valve operating.

Where does the gauge go on the water heater? At the garden hose fitting at the bottom typically used for emptying? And if so, do I need to open the fitting to get an accurate reading?

Yes and Yes.

DO NOT PUT THIS GAUGE ON THE WATER HEATER DRAIN VALVE. Any hose bib in the house plumbing system will do as the entire house's plumbing system is CLOSED. Often a laundry room sink will have a hose thread faucet.

My first thought would be the T&P valve needs replaced. May have come defective out of the box. After that I would look at a check valve between your incoming cold and the water heater, and put an expansion tank between the check valve and the water heater. Reasoning for the check valve is to stop your water heater from trying to heat the cold line. Also, check the temperature on your water heater, it may be set too high, usually it is set by the mfg at 120. Last idea is the water heater itself is heating too long even if it is set to the correct temperature. The higher the temp the higher the pressure.

"Reasoning for the check valve is to stop your water heater from trying to heat the cold line."

I was always under the impression check valves are required to prevent water reversing direction to fllow back into public supply after entering the home. The man at Lowes sold me 2 gizmos that look like 3/4tr" steel nipples with plastic liner he said would "stop heater from trying to heat lines". Cost me $15,did I even need them since I already have the whole house check valve?

I have two check valves, one for stopping the whole house backflowing to city, and one for stopping the hot water possibly going into or heating the cold line.

"and put an expansion tank between the check valve and the water heater"

I see what you are saying but doesn't the expansion tank act as a huge heat sink negateing benifit of check valve?

When you see hoof prints think horses not zebras.

Code has required back flow prevention into a municipal water system since the 90's. That back flow prevention is usually incorporated in a PRV valve at the water service entrance to the house. It can also be a simple check valve at the water service entrance if there's no PRV installed.

If one has a problem with hot water entering the cold side plumbing then there is a problem. A check valve might treat the symptom but I'd want to cure the disease.

Thermal expansion tanks must be installed on the cold side of the water heater. They are not rated for hot water. They are usually installed with a few feet of the water heater on the cold line servicing the water heater but can be installed anywhere on the cold plumbing after the PRV valve at the water service entrance.

The only time I've ever seen a failed T&P valve on a water heater is when thermal expansion repeatedly burps the T&P valve until it fails. The T&P valve on a water heater is a safety device not a service device. It's a good idea to check it by quickly opening and closing it to see if it flows and shuts off once a year when you drain your water heater.

I am not sure you know what I mean. Here is my setup.

Cold water main --> Check valve --> Cold water main --> Tee to HW heater cold side and to cold main --> check valve on HW heater cold line --> Tee to expansion tank --> HW heater

I dont see the expansion tank as a heat sink. Cold water has to pass through it and get heated in the heater anyhow. The check valve in my case is an attempt to keep any possible heat from getting to the cold distribution.

That's one way of looking at things Lurker but following Hickam's Dictum can sometimes be more intertaining. (-:

@ Vith, the second check valve in your description does nothing and is just another moving part that can fail.

@ Klem1,

I do how questions not why questions so I favor Occam's Razor and diagnosing anything (whether medical, mechanical, or chemical) is to start at step one.

1. A closed plumbing system that includes a water heater experiences thermal expansion when the water is heated. Whether that closed plumbing system is closed by a unique check valve or one incorporated into a PRV does not matter.

2. To mitigate thermal expansion a device must be installed that can accommodate that expansion when it occurs and is benign when there is no expansion.

3. If there is no thermal expansion device installed in a closed plumbing system or there is a device that is not working properly then when the heated water expands water will burp out the T&P on the WH . Klem1, that is a hoof print made by a horse.

4. The common device that accomplishes that is a thermal expansion tank installed on the cold side plumbing inside of a closed plumbing system.

T&P valves are rarely defective since they are a safety device and get serious QC at the factory. The odds of a defective T&P valve on a new water heater are mighty slim. EVERY time I've seen a T&P valve burp the cause was thermal expansion.

Double or single check valves and hot flow into cold lines does not address the OP's concerns and will not lead to solving the OP's problem.

I did note to put an expansion tank btw. Check valve might be pointless in your opinion, and you are correct he does not 'need' it and only the expansion tank will solve the problem. Personally, I would rather ensure no thermally expanded heated water can ever be capable of backing up into the cold line even though I have an expansion tank. That is just my opinion. If the pressure of the water in the water heater (and the water heater cold inlet and expansion tank) is higher than that of the city then you will get flow from the water heater into the cold distribution till the pressure equals out.

Andy and Barney once solved a stolen cow case by following boot prints leading to a neighbor's barn. They were unable to find hoofprints(cloven or otherwise) leaving owner's barn. Sneeky neighbor had put 2 pairs of his old boots on the cow and rode her home. BTW,his cow had died when her prv malfunctioned,acted like a check valve and prevented chewing her cud.

@ Vith...

"If the pressure of the water in the water heater (and the water heater
cold inlet and expansion tank) is higher than that of the city then you
will get flow from the water heater into the cold distribution till the
pressure equals out".

Not really... in a closed plumbing system with a thermal expansion tank the tank will accommodate the water volume increase when the pressure rises just a couple PSI over the static pressure.

In a closed plumbing system the increased volume of heated water has only two places to go as long as no one opens a faucet. One place is the T&P valve and the other is to force open a toilet ballcock valve. Every other valve in the plumbing system is a positive off valve.

"I would rather ensure no thermally expanded heated water can ever be
capable of backing up into the cold line even though I have an expansion
tank. That is just my opinion". You're welcome to your opinion but you're worrying about something that doesn't occur. We're not looking at gallons of water moving in the system when the water is heated. When thermal expansion burps a T&P valve you usually get an ounce of water while the pressure is relieved. Much more than an ounce moves into the thermal expansion tank as the water volume rises when the water is heated but if the tank is correctly sized and the air charge correctly set you will never get anything but cold water in the expansion tank.

If it makes you feel better you can install a dozen check valves and all that accomplishes are more connections for a leak to occur and more moving parts to fail.

Is this Amazon description the right T&P for the old one (next picture). In a Bradford White 50 gallon.

I should say I am replacing the expansion tank with the appropriate Watts tank, and replacing the T&P at the same time. I'm not sure what the pre charge should be though. 75 PSI in the house.

You don't want to get the gauge to prove you have a thermal expansion problem and are just going to add and replace stuff hoping you fix it?

Don't change two things at once. If you do, you won't know what the problem was and what fixed it.

You need the correctly sized expansion tank and the air charge needs to be correct. If you read the link in my first post the info to answer both questions is there. http://www.watts.com/pages/learnabout/thermalExpansion.asp

While T&P vales look the same they are different and Amazon is NOT the place to buy them. If you are intent on changing the T&P valve, and IMO there is nothing wrong with the one in the WH now, make sure you get EXACTLY the same T&P valve. You can call Bradford White or the local Bradford White distributor to get the correct specs but you changing the T&P valve may void the Bradford White warranty.

I'm learning my current expansion tank is non-potable. Pre charge was never changed. I discovered a Watts video that tells you to set the pre charge same as the psi in the house, which I intend to do. Probably ~75 psi. I'll order the tank alone. Here's hoping.

It might be too far along for anything new but if tank wasn't charged, expansion tank might be where the problem is. It was stated in first post that tank failed right a way and filled with water. If bladder was not inflated at installation, tank will fill with water but that isn't nessarly a failure. The track look's to be equid,definatly not bovine. Shoot low,they migfht be ridding Shetlands.

So, I did the Watts website calculation and realized 2 things:

1. My current pressure tank is non-potable, in a potable system.

2. I need a PLT-12 Watts tank. 4.5 gallons. 150 psi.

The tank is arriving today. I did 2 pressure tests:

1. Garden spigot, 70 psi

2. Laundry tub faucet, just after morning showers, 130 psi

What would you set pre charge to? Is there a max on the tank?